A normal body temperature for a baby is around 98.6°F (37°C), though it can fluctuate by about 0.9°F (0.5°C) throughout the day. A reading of 100.4°F (38°C) or higher, taken rectally, is considered a fever. Knowing what’s normal helps you spot when something is off, and the method you use to take your baby’s temperature matters more than you might expect.
Normal Range by Measurement Method
The number you see on the thermometer depends on where you take the reading. Different parts of the body run at slightly different temperatures, so each method has its own threshold for what counts as a fever:
- Rectal, ear, or forehead: Normal is below 100.4°F (38°C). At or above that is a fever.
- Oral: Normal is below 100°F (37.8°C).
- Armpit: Normal is below 99°F (37.2°C).
Armpit readings tend to run the lowest because the thermometer isn’t inside the body. That’s why the fever cutoff is lower for armpit measurements. Rectal readings are considered the gold standard for babies because they most closely reflect true core body temperature. If you get a borderline reading from the armpit or forehead, a rectal reading gives you the most reliable answer.
Why Temperature Fluctuates During the Day
Your baby’s temperature isn’t a fixed number. It naturally dips in the early morning and rises slightly in the late afternoon and evening. This daily swing of up to 0.9°F is completely normal and doesn’t mean your baby is getting sick. Activity, crying, feeding, and even a warm bath can temporarily nudge the reading up. A single slightly elevated number in an otherwise happy, feeding baby is rarely cause for concern on its own.
Which Thermometer to Use
Rectal thermometers are the most accurate option for infants. They’re more invasive, but for babies under 3 months especially, accuracy matters enough that pediatricians recommend rectal readings when you suspect a fever.
Ear thermometers are not recommended for babies under 7 months. An infant’s ear canal is small, and earwax or minor shape differences can throw off the reading. Forehead (temporal artery) thermometers are convenient but can be less accurate in certain conditions: direct sunlight, cold air, or a sweaty forehead can all skew results. If you use a forehead thermometer as a quick screen and the number looks concerning, follow up with a rectal reading.
Oral thermometers work well for older children but aren’t practical for infants who can’t hold one under their tongue.
When a Temperature Is Too Low
Most parents worry about fever, but a temperature that’s too low also deserves attention. The World Health Organization defines hypothermia in newborns as a core temperature below 97.7°F (36.5°C). Premature babies are especially vulnerable because they have less body fat and a harder time regulating their own heat.
A cold room, wet clothing, or being underdressed can cause a baby’s temperature to drop. If your baby feels cool to the touch and registers below 97.7°F, warming them with skin-to-skin contact and dry blankets is a good first step. If the temperature doesn’t come up or your baby seems lethargic or is feeding poorly, that needs prompt medical attention.
Clothing, Swaddling, and Overheating
External factors can push a baby’s temperature reading higher than their true core temperature. Heavy swaddling, too many layers, or a warm room can all make a baby feel hot and register a slightly elevated reading. Before assuming your baby has a fever, remove a layer of clothing, wait 15 to 20 minutes, and retake the temperature. If the reading drops back to normal, your baby was simply overdressed.
Overheating is worth taking seriously beyond just misleading thermometer readings. Excessive thermal insulation from heavy bedding and clothing has been associated with a raised risk of sudden infant death syndrome (SIDS), particularly when babies are bed-sharing. A good rule of thumb: dress your baby in one layer more than you’d wear comfortably in the same room.
Fever in Babies Under 2 Months
A fever of 100.4°F (38°C) or higher in a baby under 2 months old is treated as a medical priority regardless of how well the baby appears. Young infants have immature immune systems, and a fever can be the only visible sign of a serious infection. The American Academy of Pediatrics has specific guidelines for evaluating febrile infants as young as 8 days old, and pediatricians take these temperatures seriously even when a baby looks fine otherwise.
For babies older than 2 to 3 months, a fever alone is less alarming. How the baby is acting matters more than the exact number on the thermometer. A baby with a temperature of 101°F who is alert, feeding, and making wet diapers is generally in a different situation than a baby with 100.5°F who is limp and refusing to eat.
Signs That Need Immediate Attention
Fever combined with certain symptoms signals something more urgent. Watch for:
- A widespread red or purple rash
- A stiff neck paired with a severe headache (in older infants and toddlers)
- Difficulty breathing or shortness of breath
- Extreme weakness or unusual drowsiness, where your baby is hard to wake or seems confused
- Signs of dehydration: dry diapers, very dark urine, or no tears when crying
- Severe abdominal pain
If your baby has a fever along with cracked red lips, a red tongue, red eyes, and swollen hands or feet, that specific combination warrants a call to your pediatrician right away. And if your baby has a seizure lasting more than four to five minutes, call emergency services.
A seizure that lasts under two minutes, while frightening, is typically a febrile seizure. These are more common than most parents expect and are usually not dangerous, but your pediatrician should still know about it.